1. Field of the Invention
The present invention relates to a method for forming in an optical recording medium a multilayer reflection hologram serving as an information recording layer for recording information using holography.
2. Description of the Related Art
Japanese Translation of PCT International Application No. 2002-502057 discloses a system and a method for writing data by forming a format hologram (reflection hologram) in a recording medium and subsequently locally altering the format hologram to change the reflectivity for bit-by-bit recording.
In a system disclosed in U.S. Pat. No. 6,574,174B1, information is recorded in a recording medium by locally altering a format hologram, in a manner as described above, and the recording medium includes, in addition to a hologram layer, a dedicated servo layer used for servo control.
Miyamoto et al., Tech. Digest of ISOM/ODS2008, MB04 discloses that when information is reproduced while servo control is performed using a dedicated servo layer as described above, the tilt margin of an optical head is small and this leads to difficulty in achieving good reproduction.
Miyamoto et al., Tech. Digest of ISOM/ODS2008, MB04 also discloses that tracking servo characteristics with tilt margin can be obtained by providing a hologram recording layer as a micro hologram and directly tracking-servo-controlling this micro hologram itself.
As described above, the use of the dedicated servo layer during reproduction of an optical recording medium that uses a format hologram leads to difficulty in achieving good information reproduction because the tilt margin is small.
When tracking servo control (direct servo control) is performed directly on the surface of a micro hologram, a large tile margin can be obtained. However, when a plurality of format hologram layers are provided, a huge amount of time is required to form servo marks in each layer, and this causes a significant increase in the cost of manufacturing the optical recording medium.
It has been proposed that to increase the recording capacity of an optical recording medium, a multilayer stack of reflection hologram layers is formed. However, to perform direct servo control on the multilayer stack of hologram layers, the hologram must be modulated in the depth direction, and the focus servo control of recording-reproduction light must be performed.
To spatially modulate a hologram, the information recording layer may be formed as an alternate stack of hologram layers and spacer layers, or a modulated hologram may be recorded in a uniform information recording layer. However, to successively deposit the hologram and spacer layers, a deposition step is required. Unfortunately, this leads to an increase in the manufacturing cost of the optical recording medium.
To record a modulated hologram, mutually opposed laser beams are brought to interference in a hologram material to successively form localized reflection holograms. However, in this method, the area of interference fringes (reflection hologram) formed at a single time is small, so that a large amount of time is required to form the holograms over the entire optical recording medium. In addition, since areas other than the areas of the interference fringes are exposed to light, the efficiency of hologram formation is low.